In solid structures, particularly in load-bearing structures of, for example, bridges, buildings, tunnels, railways, containment walls, dams, embankments, slabs and beams of buildings, pipelines and underground structures of city underground railways, and so on, it may be particularly helpful to monitor significant parameters, such as, for example, pressure, temperature, and mechanical stresses. Such monitoring, carried out periodically or continuously, may be useful both in the initial step and during the life time of the structure.
To this aim, it is known to use electronic monitoring devices based on electronic sensors, which are capable of providing relatively good performance at relatively low costs. Usually, such devices are applied on the surface of the structures to be monitored, or within recesses already provided in the structure and accessible from the outside.
However, such devices are not generally capable of exhaustively detecting the parameters within the structure to be monitored, which may be very useful for assessing the quality of the structure, the safety thereof, its aging, the reaction to variable atmospheric conditions, etc. Furthermore, such devices can typically only be applied after the structure has been built, and not while it is being built. Therefore, these devices are unable to evaluate possible initial defects.
As a partial response to these desires, the approach shown in the U.S. Pat. No. 6,950,767, provides an electronic monitoring device that is entirely contained, i.e., “buried” within the material (for example, reinforced concrete) of the structure to be monitored. Such a device is a whole system encapsulated in a single container, including several parts that are assembled on a substrate, such as integrated circuits, sensors, antennas, capacitors, batteries, memories, control units, and still other substrates, formed in different “chips” that are mutually connected by electrical connections, for example, metal connections.
Therefore, on the whole, U.S. Pat. No. 6,950,767 discloses an approach using a “System in Package” (SiP) in which the SiP is coated in a casing of mold material, such as an epoxy resin. The casing may be a conventional package, per se known. Such a system communicates with the exterior by virtue of a radio communication subsystem included therein, having antennas of dimensions suitable to communicate with a remote system.
It is noted that a device or a monitoring system operating within a solid structure may desirably address particular operative conditions. For the present description, solid structures are considered, such as structures made of building material, for example cement, concrete, mortar.
A monitoring device or system to be initially “buried” in a building material (e.g., uncured concrete, which then cures and solidifies) and to remain then “buried” in the solid structure, is subjected to important operative conditions. Furthermore, it is in contact with a material having irregularities, from several points of view, due to intrinsic characteristics or imperfections. All of this causes at least two types of drawbacks, respectively correlated to reliability problems and to possible measurement inaccuracies, which are described below. Referring to reliability problems, considerable causes for wear are, for example, relatively high pressures, also of some hundreds of atmospheres, as well as causes related to water seepage, over time, which may damage the system.
A drawback of the known systems, such as the one described in U.S. Pat. No. 6,950,767 is that they are relatively complex systems and may be damaged due to the operative conditions in which they have to operate. In particular, the electric interconnections between the various components of the SiP described in U.S. Pat. No. 6,950,767 can be vulnerable due to the mechanical stress subjected to the SiP inserted in the structure.
Furthermore, the “window” that has to be left in the package to allow the sensor to detect the corresponding parameter may be a weak point due to possible moisture seepage. Again, a crack or imperfection of the coating material may let water penetrate within the SiP, causing short-circuits. Besides water, other substances, such as potentially corrosive acids, can penetrate.
In general, although they are designed for such use, the reliability of systems such as the one described in U.S. Pat. No. 6,950,767 has its limit in the complexity of the structure of such a system, although miniaturized, and the unsuitability of the commonly used known types of packages, due to extreme conditions.
Referring to problems of incorrect or inaccurate measurement, initially, it may be considered that the solid structure to be monitored includes a material that may not be perfectly homogeneous. For example, concrete is an artificial stone material formed of stone aggregates having different dimensions, referred to also as inert, which are bonded with cement, as a hydraulic binder activated by a chemical reaction with water. Therefore, in concrete it may be possible to classify cement granules (having a dimension ranging from 1 to 50 μm) and a wide variety of granules of inert aggregates, which, quantitatively, can account for up to 80% of the weight. The concrete inert aggregates are usually classified, based upon the diameter of the granules thereof, such as very fine, or fillers (diameter<0.063 mm); fine, or sand/grit (0.063-4 mm); coarse fine gravel/finely crushed stone, (4-15 mm); and gravel/crushed stone, (15-40 mm).
As it may be known in the field of building construction, that different types of concrete can be obtained with mixtures composed of inert aggregates of different dimensions in various percentages. Such different types of concrete have different characteristics, in terms of properties such as mechanical resistance, porosity, compactness, and lightness. In any case, to obtain a concrete that may meet minimum requirements for each of the above-mentioned properties, it may be desirable to use a mixture of inert aggregates having different granularities.
With respect to very fine inerts, microsilica or silica fume is sometimes used, which may include particles having a diameter ranging between 0.01 and 1 μm. Microsilica behaves as a very fine filler, suitable to fill the free spaces between the cement granules, thus increasing the cement compactness. On the other hand, due to the high specific surface of the microsilica particles, they cannot typically be used in percentages above 10%, which may excessively increase the slurry water amount. In other types of concrete, the fine and very fine aggregates may be present in a minimum percentage.
Therefore, it shall be noted that, at a millimetric or sub-millimetric scale, the concrete intrinsically has, due to its nature, irregularities that are randomly distributed within the volume of the solid structure it forms. In addition, there may be local imperfections.
In such conditions, a monitoring device may be considered, for example, arranged in a specific position of a concrete structure, suitable to detect a force (for example, corresponding to a mechanical stress) applied by the solid structure, at a macroscopic level, in that specific position, and along a certain direction, for example, a vertical direction. The device locally detects the force in the point of the surface of an integrated circuit, included therein, in which there is a sensor.
Such a sensor is typically sensitive to the piezoresistive effect, and it is capable of measuring a force in a determined direction, which is made to match, in the initial positioning step, to the direction of interest (for example, a vertical one). If the force, while keeping the intensity constant, is applied to a different direction, the sensor sensitivity decreases, in accordance with the piezoresistive effect, and the actually detected force turns out to be lower, sometimes significantly lower.
On the other hand, due to the above-mentioned characteristics of the concrete, the sensor buried in the solid structure may be in contact with a part of the structure locally having very different and inhomogeneous characteristics (presence or absence of micro-cavities, presence or absence of coarse particles, or co-presence of particles having different dimensions, etc.) Such particles may exert a punctual action on a microscopic scale, which may be different from the macroscopic action to be correctly detected.
In particular, it may be possible that the concrete locally exerts a force upon the sensor through particles having a variable granularity in a different direction than the macroscopic direction of the force that is to be detected. Consequently, the sensor, due to the characteristics thereof, illustrated above, detects a force intensity that is lower than the actual one.
The described example shows how, by using known devices, particularly severe measurement errors may be originated, even systematic errors. In brief, if a general known monitoring device is buried within a solid structure, with an integrated circuit without a package, inaccuracy problems may arise (or even systematic errors) during the measurements.
If a general known monitoring device is buried within a solid structure, having a package of a common type, relatively severe reliability problems may arise, i.e., high probabilities of damage over time. Also in this case, further measurement errors may originate. For example, the conventional packages can be subjected to a volume reduction following a degassing phenomena that may alter, for example, pressure measurements. Furthermore, the interface between the package material and the solid structure material may not allow such an adhesion to correctly transmit a parameter to be measured.